Electron guns employed in television CRTs generally can be divided into two basic sections (1) a beam forming region (BFR), and (2) an electron beam focus lens for focusing the electron beam on the phosphor-bearing screen of the CRT. Most electron beam focus lens arrangements are of the electrostatic type and typically include discrete, conductive, tubular elements arranged coaxially and having designated voltages applied to each of the elements to establish an electrostatic focusing field. A monochrome CRT employs a single electron gun for generating and focusing a single electron beam. Color CRTs typically employ three electron guns with each gun directing a respective focused electron beam on the CRT phosphorescing faceplate to provide the three primary colors of red, green and blue. The electron guns are frequently arranged in an inline array, or planar, although delta gun arrays are also quite common. The present invention has application in both monochrome and multi-electron beam color CRTs. A sharply focused electron beam having a small spot size provides a video image having high definition. In order to reduce beam spot size, limiting apertures of small size have been incorporated in the electron gun. These prior limiting aperture approaches have met with only limited success because of three sources of performance limitations.
In the conventional design, the limiting aperture is typically disposed in the focus voltage grid. In this region, the electrons typically have kinetic energies on the order of a few kilovolts (KV) which causes secondary electron emission at the focus grid. The secondary electrons generally land on the CRT screen causing loss of contrast and/or loss of purity in a color which generally appears as a haze surrounding a video image. Because the electron beam typically has a large cross-section in the beam focus region, the focus grid limiting aperture is also relatively large. This increases the likelihood of the secondary electrons being incident on the screen. There is usually no grid with a voltage higher than the limiting aperture and lower than the anode to absorb the secondary electrons before they reach the screen and cause loss of resolution. A second problem arises from the electrons intercepted by the limiting aperture flowing through the resistor chain toward the CRT's anode. This electron current causes focus voltage shift and a resulting de-focusing of the electron beam. The third problem also arises from the energetic electrons incident upon the focus voltage grid about the limiting aperture. Because the intercepted electrons in this high voltage region of the electron gun have high kinetic energy (the CRT gun typically has a focus voltage of a few thousand volts), the intercepted high energy electrons release their kinetic energy at the aperture region causing a substantial increase in the temperature of the focus voltage grid, which in some cases becomes vaporized before this energy can be dissipated. These three problems have limited prior art attempts to reduce electron beam spot size by means of a small aperture in the electron gun.
The present invention overcomes the aforementioned limitations of the prior art by providing a relatively low voltage limiting aperture situated at a field-free zone in the main focusing lens of an electron gun which avoids electron beam aberration, minimizes secondary electron emissions, does not adversely affect electron beam focusing, and intercepts the peripheral electrons at a relatively low energy to minimize grid thermal dissipation.